1. Field of the Invention
This invention relates generally to the field of integrated circuit packaging.
2. Description of the Related Art
In the manufacture of integrated circuits, extremely fine circuits are photolithographically placed onto a chip, also called a die. The circuits on the chip terminate at conductive terminals on the face of the chip, which must be electrically connected to power and to other chips. For protection of the chip and ease of handling, these chips are bonded to a substrate and to electrical connections ultimately leading to conventional printed circuit boards. One attractive way of connecting the conductive terminals on a chip is where the terminals are distributed over the face of the chip and connected to a circuit substrate by conductive paste or solder balls in a pattern of “C4” connections.
In some integrated circuit packaging applications, there are at least two integrated circuit chips having very high communication bandwidth or bitrate between them. For example, a high speed processor often requires a very high communication bandwidth with an associated memory chip. A cost-effective, high bandwidth interconnection between two (or more) devices can be done by stacking devices with the C4 patterns facing each other and electrically interconnecting them vertically using very short connections between the chips. This approach ensures a consistent, very short length interconnect between the devices which can enable extremely wide, high speed, low skew busses between the two devices.
Where chips are stacked face to face, an interposer between the devices is usually necessary to efficiently distribute power to the devices and facilitate high yield assembly and test. Ideally, the interposer should be no thicker than required to adequately distribute power to the devices since the thickness of the interposer increases the interconnect length of the busses between the devices and effectively degrades the electrical performance of the interconnect.
Traditional alumina ceramic substrates have been tried as the interposer; however, the screened tungsten and/or molybdenum crosshatched conductor patterns employed for power planes in alumina constructions offer relatively high resistance. As a result, numerous power planes must be employed in the design to facilitate efficient power distribution, which results in a relatively thick interposer. Glass ceramic constructions employing copper paste instead of molybdenum or tungsten, which offer significantly lower resistance per conductor layer and therefore potentially thinner interposer constructions, have also been tried. However, cost and availability of the glass ceramic construction make it less desirable for many high volume, low cost markets.
Low cost, organic substrate constructions based on sequential build-up of fine pitch, thin wiring layers over a rigid, drilled printed circuit board (PCB) core represent a lower cost approach. However, it is difficult to cost-effectively drill the rigid PCB to support the C4 pitch necessary for most interposer designs geared for high bandwidth. Although more sophisticated, multi-layer organic structures exist for high performance, niche applications that can support the necessary vertical interconnect density (for example, laser drill density), such as IBM HyperBGA®, these substrates are typically even more expensive than the ceramic substrates cited above.
Bumped, two metal layer (2ML) flex circuits can be used as an interposer. As used herein, “2ML flex circuits” includes circuits having an intermediate insulating layer generally thinner than about 75 microns. However, bumped 2ML flex circuits require a stiffener bonded to the flex circuit, adjacent to the location of the integrated circuit die. Commonly used single-sided stiffeners in these constructions must be rigid enough to overcome the tendency of an unbalanced lamination to warp. This typically requires high modulus metal stiffeners having a thickness in the 500-1000 micron range.